This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Siderophores are bacterial products which bind iron and increase the rate of bacterial iron transport. The results of siderophore function are best observed in low-iron media as a more rapid onset of growth and possibly an increase in growth rate. Some iron-binding compounds are also known to function in media containing animal or human serum. Since an extension of this phenomenon is the stimulation of bacterial growth during infections, siderophore synthesis has been linked to bacterial virulence. Pseudomonas aeruginosa produces two siderophores under iron-limiting conditions, pyoverdine (PVD) and pyochelin (PCH). Vanadyl sulfate, at a concentration of 1[unreadable]2 mM. which is well-suited to observation via EPR, strongly inhibited growth of P. aeruginosa PAO1, especially under conditions of severe iron limitation imposed by the presence of non-utilizable Fe(III) chelators. In order to elucidate the role that vanadyl plays in the delayed onset of virulence attendant on vanadyl interference with iron uptake, we are initiating in vitro studies of vanadyl siderophore chelates at X and Ka band under physiological conditions. Vanadyl is a well-known biologically relevant EPR active spin probe and can report on siderophore dynamics when chelated that are inaccessible to EPR of iron siderophore chelates, due to the rapid relaxation of ferrous and ferric ions.